Basic components of modern plasma arc torches include a torch body, an electrode (e.g., cathode) mounted within the body, a nozzle (e.g., anode) with a central orifice that produces a pilot arc to the electrode to initiate a plasma arc in a flow of a suitable gas (e.g., nitrogen or oxygen) and associated electrical connections and passages for cooling, and arc control fluids.
In piercing metal using a plasma arc torch, an important design consideration is the ejection of molten metal from the cut kerf back onto the torch which can destroy the nozzle. There are two principal modes for this destruction. First, molten metal ejected from the cut kerf can disturb the plasma jet causing it to gouge the nozzle. Second, the molten metal can solidify and adhere to the front face of the nozzle, which eventually causes an electrical bridging between the nozzle and the workpiece. This results in “double arcing” which can drastically reduce the life of a nozzle.
There have been several approaches to solving the gouging and double arcing problems created by the ejection of molten metal. In high current plasma cutting torches (e.g., 200 amperes and more), the solution has been to use a multi-piece nozzle with water injection cooling. A typical such nozzle of the type manufactured by Hypertherm, Inc. corresponding to Hypertherm Models HT400 and PAC500, the front face of the nozzle is made of a ceramic. This arrangement controls gouging and double arcing because (1) the ceramic nozzle face is non-conducting and therefore will not cause double arcing and (2) the nozzle is protected by the ceramic barrier. Further, the excellent cooling properties of the water, operating by cooling the ceramic nozzle piece and by water vapor cooling the molten metal ejected during piercing, inhibit the molten metal from bonding or fusing to the ceramic element or in the extreme case, from attacking the ceramic. A variation on the high-current, multi-component nozzle similar to the nozzle sold by Hypertherm as its Model PAC500, is a ceramic nozzle piece incorporating radial water injection, but the ceramic nozzle piece is replaced by a copper front piece. An insulating element separates the nozzle components so that the front of the nozzle is floating electrically. The copper is more readily cooled than the ceramic and it withstands abuse significantly better, and therefore has a longer life.
In some cases, a ceramic insulating sleeve is attached to the outside of the nozzle in an attempt to protect the nozzle. This is a so-called “shield cup”. Its main purpose is to stop nozzle-to-workpiece contact. An operator can touch or drag the torch on the workpiece without double arcing. This ceramic sleeve, however, offers little or no protection during piercing against molten metal splatter and the attendant gouging and double arcing problems. Also, the ceramic shield (1) can be brittle and can break easily and (2) as a result of not having the protection of water cooling, can be attacked by the molten metal ejected from the cut.
Cooling consumables (e.g., shield) of a plasma arc torch with a cooling liquid (e.g., water) can have safety benefits. Without liquid cooling, the consumables can reach extremely high temperatures that can pose a safety issue during use. A lossless cooling system allows the use of a dry plasma and a dry cutting table. Dry tables can be desirable due to the reduced mess and elimination of the need to dispose of the used/contaminated water, which can be considered to be hazardous waste.